Efforts aimed at miniaturization have always been a strong component associated with the advancements in the medical device/implant industry. In the 21st century, this trend continues to be ever-present with a drive to scale down devices from a cubic centimeter range down to a cubic millimeter range. Smaller scale medical devices demonstrate great potential for reducing healthcare costs and mitigating trauma associated with invasive implant surgeries, while concomitantly improving both post-operative medical evaluations and convalescence periods.
The benefits of miniaturization are demonstrated with the emergence of a new class of cardiac pacemaker devices that are small enough to be inserted directly into a patient's right ventricle. The size reduction of these pacemakers and other medical devices is limited, in part, by the power sources that fuel the device's operation. Pacemaker batteries typically consume up to 40-80% of the device's volume. Consistent with this notion, further significant reductions in the scale of medical devices have been limited by the power sources themselves. While device components and electronic circuitry can be reduced to ever more diminutive dimensions, battery technology has traditionally remained limited to cubic centimeter dimensions arising as a consequence of exponential losses in energy density and capacity as batteries approach cubic millimeter scales. However, recent advancements in both electronics and battery technologies have led to reduced system power demands and higher power outputs from more diminutive power solutions.
Recent academic teams have laid the groundwork for an implantable cubic millimeter-scale device that can measure a glaucoma patient's intra-ocular eye pressure. This tiny device comprises a CMOS microcontroller that contains an on-board radio transmitter and sensor with cubic millimeter dimensions. The device operates at ultra-low power levels, e.g. picowatt power range during sleep cycles and at approximately microwatt power range during operational periods. Due to the extremely small dimensions of the device, it can be directly implanted within a patient's eye. The device can measure/record intra-ocular pressure throughout the day and radio-transmit processed data at periodic intervals to an external device for analysis.
This millimeter scale device utilizes an exceedingly small-sized solar cell to trickle-charge a millimeter scale LiPON (lithium phosphorous oxynitride) battery to provide a rechargeable power source. Such LiPON batteries are available from Cymbet Corporation of Elk River Minn., a manufacturer of small thin film, solid state secondary batteries. The Cymbet LiPON battery has approximately a 1 to 50 microamp-hour capacity, and the duty cycle of the device allows it to operate indefinitely as long as the solar cell provides an average of 10 nanoamps of current to the LiPON battery.
This configuration is well-adapted to an ocular implant where visible spectrum light energy is easily accessible and can be transmitted through the eye thereby providing the solar cell with the necessary trickle-charge power for the LiPON battery. However, such a solution is not suitable for implants that are not accessible to visible light, thereby rendering the solar cell component moot. An example of such a scenario might be a sensor embedded within a tumor for measuring pressure changes associated with tumor growth.
It should be noted that the LiPON battery can at best provide a maximum of 28 days of operation for implants in a stand-alone operation; this is far too short for most implantable devices that are not accessible to visible light sources. Clearly, a long-term (light-independent) trickle-charging source is required for maintaining the LiPON battery and subsequently facilitating operational effectiveness of such small scale sensor systems operating with such a LiPON battery.
Unfortunately, chemical batteries with less than cubic centimeter dimensions have a less than optimal energy density. The optimal energy density for a lithium iodide battery in conventional pacemakers is approximately 1 watt-hour per cubic centimeter with a volume greater than about 3.0 cubic centimeters. To supply sufficient power to shrinking medical devices, an energy density of greater than about 1 watt-hour per cubic centimeter is desirable for battery volumes of approximately 0.5 cubic centimeters or less. In battery volumes of 0.1 cubic centimeters, a 10 watt-hour per cubic centimeter energy density would be highly desirable due to the loss of energy capacity in such a small volume (i.e. 1 watt-hour of capacity in a tenth of a cubic centimeter).
Furthermore, the battery needs to provide power in ranges from nanoamps to milliamps to accommodate duty cycle power requirements of medical device electronics. For example, wireless signals (to transmit the sensed values to an external device) require higher power bursts for short durations while the microcontroller's sleep power provides a low, but continuous drain. An ideal medical implant power source will have a high energy density that is robust under a wide-range of power drains while having a diminutive form factor. Unfortunately, currently available betavoltaic power sources with less than 1 cubic centimeter dimensions will have only nano-watt to low-microwatt power outputs falling short of the required, or at least desired, higher power output necessary in some medical implant functions (e.g. radio telemetry, defibrillation etc.).
Historically, betavoltaic power sources for medical device implants are not a new concept as they have a demonstrated potential for high energy densities well beyond conventional chemical batteries. Betavoltaic power sources do, however, suffer from low power densities and radiation shielding requirements. In the early 1970's, a group of researchers at Donald W. Douglas Laboratories of Richland, Wash. (led by Dr. Larry Olsen) invoked a promethium-147 radio-isotope to fuel a betavoltaic power source (also referred to as a Betacel) for energizing a cardiac pacemaker, which was successfully implanted in over 100 patients. Although the Betacel's size approximated 1.0 cubic inch due to shielding requirements incurred from an associated gamma radiation emitting promethium-146 component, the successful implementation of this technology demonstrated the feasibility of betavoltaics for use within medical implants.